The '81 has Mikuni BS34 CV carbs, the '78 used VM type carbs. You would have a project on your hands switching over to the different rubber carb boots that mount to the head and most likely the boots on the airbox too. Plus the throttle cable. It would be a whole lot easier to just find a set of the proper carbs for your bike, or at least a set of VM's. I've got a fully restored set of VM26's from a '77 looking for a new home (hint hint).
